1. Field of the Invention
The present invention relates to an optical disc apparatus, a camera apparatus, and a method for controlling a light emission operation. More particularly, the present invention relates to an optical disc apparatus, a camera apparatus, and a method for controlling a light emission operation, wherein power consumption and an increase in temperature due to a laser light emitting portion mounted on an optical head portion are suppressed.
2. Description of the Related Art
Recently, the development of a technique for writing data onto a phase change rewritable optical disc using a blue laser and an objective lens having as high a numerical aperture (NA) as 0.85, has proceeded closer toward an optical disc having a diameter of as small as about 5 to 8 cm, which can store as large as about 4 to 16 GB of data (the maximum volume is achieved by a double layer technique). Such an optical disc can store video and audio data with the same quantity and quality as those of typical cassette tapes, if an image compression technique, such as MPEG2 or the like, is used. Therefore, a full-fledged optical disc video camera would be realized. If a transfer rate is increased, such an optical disc can be used to store high-definition (HD) video images.
Optical discs have an advantage of random access, i.e., no rewinding, which may cause irritation of the user, is required. For example, by selecting thumbnail images, a desired scene can be quickly reproduced. Thus, optical discs lead to the realization of a video camera convenient and comfortable to the user. In addition, still pictures compressed by JPEG or the like can be stored on the same optical disc. Such a video camera is highly expected as the next generation of video camera to provide a new concept to the user.
Generally, a laser light emitting portion incorporated in an optical disc apparatus has a greater quantity of heat generated per volume than other ICs or electronic components. The operation of such a laser light emitting portion is guaranteed below as low as about 60° C. to 70° C. (maximum temperature), and the life span is shortened as the operating temperature is increased even if it is below the maximum temperature. Therefore, there is a demand for an optical disc apparatus, in which the power consumption of the whole apparatus is reduced or the heat dissipation structure is improved to suppress an increase in temperature inside the apparatus, thereby reducing the operating temperature of a laser light emitting portion mounted on an optical head. The present invention is directed to the reduction of power consumption, particularly the reduction of the power consumption of the laser light emitting portion itself. By suppressing the power consumption of the laser light emitting portion, it is possible to suppress an increase in temperature of the laser light emitting portion under the same ambient temperature (temperature inside the apparatus).
A conventional technique for reducing the power consumption of a laser light emitting portion, is intermittent reproduction for music MD (e.g., Japanese Laid-Open Publication No. 7-161043). This is a method for starting or stopping an optical disc apparatus at intervals during a reproduction operation. According to this technique, substantially no power is consumed by the laser light emitting portion and the like during the stop period, thereby making it possible to reduce the average power for an optical disc driving portion including the laser light emitting portion. Therefore, the life span of the battery can be extended.
Hereinafter, the above-described intermittent reproduction operation will be described. It is assumed that the reading rate of music data from an optical disc is much higher than the input rate of music data into a decoder. To reproduce music, an optical disc driving portion is actuated. Music data is rapidly reproduced and is transferred and temporarily stored in a buffer memory (reading state). If the buffer memory is full, the optical disc driving portion is temporarily stopped so that substantially no power is consumed by the optical disc driving portion, the laser light emitting portion, and the like (stop state). Meanwhile, the music data is transferred from the buffer memory to a decoder at a constant rate to continuously reproduce music. Subsequently, if the remaining quantity of the music data goes below a prescribed level of the buffer memory, the optical disc driving portion is operated again and the continued music data is rapidly read out and stored into the buffer memory. The above-described operations are repeatedly performed during the reproduction of music. The music data is transferred to the decoder at a constant rate during the reproduction of music.
During the reading state the optical disc driving portion consumes power for actuating and reading operations, while during the stop state substantially no power is consumed. Therefore, the average power consumption can be reduced, depending on the ratio of the duration of the reading state (read time) to the duration of the stop state (stop time). Although the laser light emitting portion is continuously on during the read time, it goes to the stop state before a significant increase in the temperature of the laser light emitting portion. Therefore, the laser light emitting portion is cooled during the stop state, thereby making it possible to reduce the average temperature of the laser light emitting portion.
Note that the actuation from the stop state to the reading state requires initiating the rotation of a motor, switching a laser ON, focusing control, tracking control, and the like. This can take several seconds. These operations also require power slightly greater than that required during the reading state. However, when music data is reproduced from MD or the like, the stop time is much longer than the read time. Therefore, power consumption can be reduced and an increase in temperature of the laser light emitting portion can be suppressed. Note that the intermittent reproduction technique is also applicable when data is recorded or written onto an optical disc.
On the other hand, a technique has been devised for reducing the power consumption of a laser light emitting portion during reproduction where an optical disc is driven. For example, a technique for pulse-driving a laser light emitting portion in synchronization with the periods of reproduced data signal (Japanese Laid-Open Publication No. 2002-63726), or a technique for pulse-driving a laser light emitting portion with a frequency much higher than the frequency of data signal (Japanese Laid-Open Publication No. 2001-331960), have been disclosed. In the former, laser light is intermittently emitted, matching the channel bit periods of data. Data edge is detected when light is emitted, while an edge is not detected when light is not emitted. Therefore, the power consumption of the laser light emitting portion can be reduced with substantially no influence on information reproduction. In the latter, the laser is alternately switched on and off at a rate much higher than the frequency of data, and data is reproduced by detecting a data envelope. In this case, an influence of the flickering of the laser can be removed and the average current of the laser light emitting portion can be reduced, thereby reducing power consumption.
However, when an optical disc apparatus is used for a system requiring a high speed data processing rate (e.g., a video image system), the effect of the above-described intermittent reproduction technique (hereinafter referred to as a stop-type intermittent operation) is small. This is because the ratio of the image data processing rate to the image data recording/reproduction rate to an optical disc cannot be greatly increased as compared to the ratio of the audio data transfer rate to the image data transfer rate. As a result, a longer time has to be assigned to reading data from an optical disc during a prescribed period, while a shorter time is assigned to the stop period. Therefore, the effects of the stop-type intermittent operation, such as a reduction in power consumption and the suppression of an increase in temperature, are reduced.
When the data processing rate is increased closer to the data recording/reproduction rate, it is difficult to perform a stop operation itself, because of the time from the stop to the restart. That is, it is difficult to perform stop-type intermittent operations.
A specific example will be described below. It is assumed that a small-diameter optical disc has a limitation on the number of revolutions, so that the maximum transfer rate is limited to, for example, 20 Mbps. To realize an audio and video-mixed stream having image quality comparable to that of DV tape using MPEG2, the data stream needs to be transferred at a rate of 9 Mbps. If the stream is transferred at such a rate, intermittent operations can be performed. However, the read time includes the actuation time of a few seconds. Therefore, it is possible to realize intermittent operations having a period of several tens of seconds. However, it is difficult to increase the stop time by a factor of ½ or more. Taking into account a spare time for retrying for actuation or the like, the stop time needs to be further shortened. Therefore, in this application, a satisfactory intermittent effect cannot be expected. If the stream is transferred at a so-called variable bit rate so as to improve image quality, the transfer rate of the stream may be, for example, 15 Mbps at maximum despite the average 9 Mbps. It is possible that the 15 Mbps transfer is continued for several tens of seconds. Taking into account the above-described spare time, it is difficult to perform a stop-type intermittent operation. Thus, in applications, such as a video camera and the like, there is a demand for novel measures for low power consumption (and prevention of an increase in temperature).
In this regard, the above-described techniques of Japanese Laid-Open Publication No. 2002-63726 and Japanese Laid-Open Publication No. 2001-331960 are effective for achievement of low power consumption. The flickering duty of a laser light emitting portion during reproduction needs to be about 50% and therefore, power consumption is expected to be reduced to such a level. However, as described in Japanese Laid-Open Publication No. 2002-63726, the technique of flickering at a data channel frequency is not considered to be practical, because when a blue laser is used to reproduce hyper-density data, more information is loot when the laser is switched off, so that the quality of reproduced signals is not practical. For example, if jitter is large, data loss is expected to occur.
Japanese Laid-Open Publication No. 2001-331960 discloses a technique for using a clock signal having a frequency sufficiently higher than a data frequency to drive a laser light emitting portion, thereby reducing the power consumption of the laser light emitting portion by half. In a light emission system which is operated at a frequency higher than a data frequency, a high frequency module (HFM) for reducing noise caused by returning light of the laser light emitting portion using current convolution may be used. However, when a laser light emitting portion is operated at a high speed comparable to the operation of HFM and a current amplitude higher than that of HFM, there is a fear that a means for driving the laser light emitting portion generates heat. Since the laser light emitting portion is driven at a high frequency, it is necessary to place the driving means close to the laser light emitting portion in order to avoid the undesired radiation problem. In this case, the heat generated by the driving means leads to an increase in temperature of the laser light emitting portion. Thus, it may be difficult to obtain an effect of suppressing the temperature increase.